The long-term objectives of my research team are to understand cell fate decisions, with emphasis on the fundamental and deeply conserved germ versus soma decision. Germ cells must remain totipotent and immortal in order to produce eggs and sperm and entire new organism's generation after generation, while somatic cells must lose those properties in order to serve their specialized roles during the finite life of an organism. W seek to elucidate mechanisms that specify germ cells, protect germline fate in those cells, and antagonize germline fate in somatic cells. Our studies combine powerful genetic, genomic, and molecular approaches in the model system Caenorhabditis elegans. My research group has established new paradigms for how the germ-soma decision is regulated. We found that during embryogenesis, the chromatin regulator MES-4 epigenetically transmits the 'memory of germline' from parental to progeny germ cells. MES-4 can promote germline development even in somatic cells of embryos, but the synMuv B chromatin regulators antagonize germline fate in the soma. At later stages, as germ cells develop, germline-specific 'germ granules' protect germline fate by antagonizing somatic fate. The specific aims of this proposal are to elucidate the underlying mechanisms by which MES- 4 promotes germline fate, synMuv B proteins antagonize germline fate, and germ granules protect germline fate. In Aim 1, we will test if transcription in the maternal germ line is necessary and sufficient to initiate MES-4 marking of genes, if MES-4 marking is propagated during embryogenesis by MES-4 binding the chromatin modifications that it generates, and if MES-4's essential role in the primordial germ cells is to guide establishment of the proper program of gene expression. In Aim 2, we will analyze the locations of synMuv B factors compared to MES-4 across the genome in embryos, and determine whether synMuv B factors block expression of germline genes in somatic cells by altering MES-4 marking of genes or by altering global chromatin organization in somatic cells. In Aim 3, we will test our model that adult germ cells occasionally misexpress somatic transcripts and that germ granules protect germline fate by repressing translation of those transcripts, and we will determine whether germ cells that lack germ granules resemble mammalian teratomas and can develop into a variety of somatic cell types in addition to neurons. Germ cells and stem cells share the special properties of immortality and pluripotency, and both are targets of regenerative medicine. The ability to harness the proliferative potential of germ and stem cells and direct them to develop into desired tissues requires knowledge of factors and mechanisms that promote appropriate cell fate decisions and that block inappropriate cell fate decisions. Our proposed studies in C. elegans wil reveal how germ cell fate is specified, protected, and antagonized by factors and processes that are conserved across species.